Methods of controlling the indoor climate in a room have been known for a long period of time. Primarily, these systems include air conditioners that have thermostats to control the operation of the air conditioner using a dry bulb temperature. A typical controller in air conditioning mode causes the air conditioning to begin operation when the temperature rises above the set point value. The air conditioner responds by injecting cold air into the enclosure until the temperature within the enclosure has fallen to a point below the set value.
It is also well known that an air conditioner removes humidity from the air as well as cools it. Typically, in order to remove humidity from the air, prior art systems must lower the temperature of the air less than the current due point temperature, the temperature at which water condenses the air.
However, with this system there are situations where humidity levels are still too high, resulting in an uncomfortable space.
Attempts to remedy this problem have not been totally successful.
For example, previous attempts to control the relative humidity in enclosures have been made by simply adding a relative humidity sensor to the thermostat and then controlling the air conditioner to hold the relative humidity within the selected point range. A problem with this approach is that the relative humidity of the enclosure air may actually rise as the air is cooled and dehumidified within the enclosure. This is because the relative humidity is a function of both the amount of water vapor in a given volume or mass of air and its dry bulb temperature. Relative humidity for any volume of air is defined as the ratio of the partial pressure of the water vapor in the air to the vapor pressure of saturated steam at that temperature. Since the vapor pressure of saturated steam drops rapidly within a temperature, a relatively small amount of water vapor and volume of air at a lower temperature can result in 100% relative humidity. Thus it is possible to have a run-a-way situation where the humidity control function in a thermostat continues to call for further dehumidification, and as the temperature within the enclosure falls, relative humidity rises and locks the air conditioning on.
Subsequent attempts to solve the problem of high humidity have involved controlling the dew point temperature of enclosure air independently of the dry bulb temperature. See U.S. Pat. No. 4,105,063 to Bergt and U.S. Pat. No. 4,889,280 to Grald and MacArthur, both patents being incorporated herein by reference. However, these systems are deficient in that the achieved enclosure temperature is not always comfortable, and having a potential for over-cycling of the cooling system. Additionally, none of the references listed above provide dehumidification after the dry-bulb temperature set point has been achieved.
Other climate control systems have included using a humidity sensor, and a dry bulb temperature sensor in the enclosure. See U.S. Pat. Nos. 5,737,934 and 5,675,979. Control of humidity using a reheat system which re-heats chilled air in order to keep the dry bulb temperature of an enclosure to a specific set point is disclosed in U.S. Pat. No. 6,012,296. Another invention on the subject of temperature and humidity control has emphasized using the numerically larger of the dry bulb and humidity temperature errors. An indoor climate controller system adjusting both dry-bulb temperature and wet-bulb or dew point temperature in an enclosure is disclosed in U.S. Pat. No. 5,346,129 and is incorporated herein by reference.
Additionally, U.S. Pat. No. 6,557,771, incorporated herein by reference, discloses a system that has a controller that continuously monitors the dry bulb temperature error and the humidity temperature error within the enclosure and controls the ON/OFF status of the cooling device based on the following criteria: a) if the humidity temperature error is less than or equal to zero, the dry bulb temperature error is used in a conventional PID (proportional, integral, derivative) control block to control the ON/OFF status of the cooling device, modifying the enclosure temperature and humidity; or b) if the humidity temperature error is greater than zero, the dry bulb temperature error is ignored regardless of its magnitude and the humidity temperature error is used in a conventional PID control block to control the ON/OFF status of the cooling device; or c) if both the humidity temperature error and the dry-bulb temperature error are less than zero, the numerically larger of the humidity temperature error and the dry-bulb temperature error is used in a conventional PID control block to control the ON/OFF status of the cooling device. In the system of '771, both the humidity temperature error and the dry bulb temperature error use the same PID control block and controller gains to prevent any sporadic equipment operation.
Other prior art attempts include U.S. Pat. No. 4,105,063, incorporated herein by reference, which uses a system that obtains its reheat from supermarket freezers and is mixed into the central heat and air units to dehumidify the air along with supplemental electric heaters.
U.S. Pat. No. 4,876,858 discloses a dehumidification system of a variable volume chiller in commercial buildings using chilled water.
The present inventions is advantageous over the prior because it meets design conditions in proportion of sensible and latent heat loads from minimum to peak load with, in certain embodiments, sensible control having priority using the refrigeration cycle of reheat.
In a conventional cooling system entering air to the cooling coil operating at a given temperature will remove both sensible and latent heat. Residential air conditioning systems are controlled purely on room temperature, a measure of sensible heat, and therefore relative humidity is only changed as a by product. It is not controlled independently of temperature. This is particularly noticeable in humid climates in off peak conditions. The exterior dew point is highest early in the morning. The temperature in the enclosure may not be cold enough to trigger the air conditioning system, but the enclosure is more humid (and thus less comfortable than desired.
In view of the above, it is apparent that there is a need to provide a more reliable and efficient system for controlling a climate, including the control of temperature and humidity. The present invention meets that need.
The capacity of the air conditioner is designed for operation during the few hours of peak time. At lower temperatures the air conditioner will cycle and operate at less than full potential. With shorter cycles, the cooling coil (evaporator) does not have time for the temperature to fall to the dew point in a short cycle. For example, when the central air conditioning system stops cooling, the moisture collected on the evaporator, evaporates back into the indoor space with the extended Blower Cycles of today.
It is important that the air conditioner be sized to achieve the longest run times possible to bring humidity to acceptable levels in the building space.
Most of the cooling season, the cooling loads are well below the capacity of properly sized air conditioners and for oversized units, short cycling is a substantial problem. The building space becomes cold and clammy.
Without being bound by theory, the basic dilemma stems from trying to control two variables, temperature and relative humidity with just temperature control. The humidity level is a moving target that is only hit during peak design conditions resulting in indoor humidity levels above 60% relative humidity, a level recommended to control microbial growth.